This invention relates to the method of making micromachined devices and semiconductor devices. Specifically, it deals with an isotropic etching method for forming structures on substrates such as silicon, glass or metal, and especially dome-shaped structures.
Bulk-micromachined devices are formed by etching a substrate such as silicon, glass or metal. There are two broad categories in bulk micromachining: isotropic etching and anisotropic etching. In isotropic etching, the etchant removes a substrate at a same rate in all directions, independent of the crystal planes, and produces a spherical cavity. This isotropic etching removes the substrate horizontally under an etch mask, and there is a significant amount of undercut beneath the etch mask. Usually wet chemical etchants are used for isotropic etching of silicon. In wet isotropic etching, samples with patterned etch-mask are immersed in an etching solution for a given time. The etching solution removes only the exposed areas. FIG. 1 illustrates a spherical cavity 100 formed by a wet chemical isotropic etching in a silicon substrate 102. The cavity profile and the etch rate are controlled by the etching condition and etchant composition.
The isotropic etching of silicon produces a spherical cavity with excellent sphericity and minimal surface roughness. Usage of a single-crystal silicon which has very few imperfections offers the advantage of an IC-compatible batch process, and the manufacturing cost could be low. For these reasons, chemical isotropic etching methods have been explored to fabricate micromachined devices, semiconductor devices, acoustic lenses for scanning acoustic microscope, optical lenses, and fusion target However, due to absence of an appropriate etch stop, the isotropic etching gives low lateral resolution and poor dimensional control of the etch cavity. Further, the fabrication of a relatively large dome-shaped structure with good sphericity and smoothness in a silicon substrate has not been easy. Earlier efforts resulted in poorly defined boundaries for the cavity. On the other hand, anisotropic etching offers finer lateral resolution and dimensional control; but anisotropic etching limits obtainable cavity shapes to rectangular shapes.
According to the present invention, a silicon wafer in which the spherical cavity or cavities shall be formed has a layer of tape pasted on a surface to be etched. The tape is preferably a polyethylene tape with a pressure sensitive adhesive. The tape will function as an etch mask.
The tape is patterned, defining a circular opening where the spherical cavity is to be formed. Then the silicon is etched in an isotropic silicon etchant to form spherical etch fronts; because of the tapes resistance to the etchant, the etching step is self-limiting. The etching is followed by dissolving or otherwise removing the tape.
An additional isotropic etching is used to improve the circularity and surface roughness of the etch front. This is achieved by providing a silicon-nitride film on the silicon substrate surface, preferably prior to taping. After the second isotropic etching, the tape is removed.
Using the self-limiting etching phenomenon in an isotropic etching, the dimensions of the desired spherical cavity can be obtained precisely by a photolithographically delineated pattern of the tape. The lateral diameter (at the top surface of the silicon wafer) of the spherical etch front (obtained after the self etch stop) is a function of the mask opening size, independent of the etching time. By keeping the etching condition the same, the lateral diameter of the spherical cavity is reproduced within 4% from sample to sample.
The silicon substrate example is one that has been tested, as described below; the method and use of the tape is also applicable to other substrates such as glass, metal and ceramics.